The invention relates to circuitry for sensing current flow through a semiconductor power switch, for disabling gate or base drive thereto, to protect the semiconductor power switch.
The present invention arose during continuing development efforts directed toward utilizing the physical interconnection structure and circuit assembly shown in pending application Ser. No. 07/120,632, filed Nov. 13, 1987, now U.S. Pat. No. 4,818,895 issued Apr. 4, 1989 entitled "Direct Current Sense Lead". Electrical circuit assemblies typically include an electrically insulating thermally conductive substrate, such as ceramic, having a plurality of electrically conductive lead frames, such as copper, mounted on the substrate, and various semiconductor elements mounted on the lead frames and enclosed by an electrically insulating housing, providing a module. The lead frames extend externally of the housing module. Examples of such circuit assemblies are shown in U.S. Pat. Nos. 3,958,075, 4,156,148, 4,196,411, 4,215,235, 4,218,724, 4,250,481, 4,266,140, 4,394,530, 4,449,165, 4,449,292, 4,488,202, 4,498,120, 4,546,410, 4,546,411, 4,554,613, 4,574,162, 4,577,387, 4,630,174, 4,700,273, 4,713,723, 4,724,514, 4,739,449, incorporated herein by reference. In other circuit assemblies, such as type TO-218, the ceramic substrate is deleted from the bottom of the housing, and the lead frame pads are left exposed when supplied to the user. The user mounts the package to a ceramic or other substrate according to his particular application.
The above noted U.S. Pat. No. 4,818,895 involves the physical interconnection structure in such circuit assemblies for gate return referencing, and also for current sensing applications. The latter enables current mode control of switched mode power supplies, which has been recognized as a better approach than voltage controlled methods, as noted in "Current Sensing HexSense Power MOSFETs Simplify SHPS Designs And Lower Losses", Sean Young, PCIH (Power Conversion and Intelligent Motion Magazine), July, 1987, page 76-83. As noted in the Young article, current mode control has advantages of improved stability, automatic feed forward compensation for input voltage variations, pulse by pulse current limiting, and ease of paralleling supplies. The current mode control approach has become popular due to a variety of integrated circuits available to handle the control functions. A disadvantage to the current mode control has been the lack of an efficient means of monitoring instantaneous values of currents in the switching devices.
A noted in the Young article, in the past the current sensing function was usually done using either a sensing resistor or a current transformer in series with the switching device. The disadvantage of the series resistor is that it must always handle energy-resulting in high heat dissipation, and its ohmic value must be chosen as a compromise between keeping such dissipation low, while at the same time generating a large enough signal. The disadvantage of the current transformer approach is that it is a magnetic component prone to saturation.
The Young article notes a current sensing power MOSFET component, the HexSense, which provides current sensing with negligible electrical losses. Such components are identical to standard power MOSFETs except that current from a few MOSFET cells are diverted to a separate source pin providing a known ratio of total current. Another pin, known as the Kelvin source, is connected to a point on the main source metallization. This Kelvin connection is the return point for the sense circuit. The voltage drop across the Kelvin pin is negligible and is unaffected by the magnitude of the main source current. This arrangement avoids errors in current sensing accuracy that would result if a voltage drop existed between the return point and the source metallization.
The present invention provides current sensing circuitry which may be connected to the structure in the above noted U.S. Pat. No. 4,818,895 and to other structures and circuit assemblies. The semiconductor chip has a pair of main terminal contact metallization pads, one of which is preferably mounted in electrical contact on a main terminal lead frame, and the other of which has a conductor jumper bond wire directly connected thereto within the housing module. The bond wire is connected to another main terminal lead frame within the housing module, and both lead frames extend externally of the housing module. The sensing circuitry includes amplification circuitry having a pair of inputs connected to the noted bond wire at spaced points therealong within the housing module and sensing current flow through the bond wire by sensing voltage between such points. The inputs are connected to an existing bond wire at the spaced points without insertion of any additional series resistance in the bond wire between the points.
The sensing circuitry includes filters rejecting high frequency noise, and filters preventing oscillation. The circuitry also includes a temperature sensor to provide thermal protection in combination with overcurrent protection. Gate or base drive for the semiconductor power switch is disabled in response to either overcurrent or overtemperature. The circuitry is fast acting and disables gate or base drive before damage occurs to the semiconductor chip.